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Abstract

This document describes the structure, content, construction, and
semantics of language tags for use in cases where it is desirable to indicate
the language used in an information object. It also describes how to register
values for use in language tags and the creation of user defined extensions for private interchange.

1. Introduction

Human beings on our planet have, past and present, used a number of languages.
There are many reasons why one would want to identify the language used when
presenting or requesting information.

User's language preferences often need to be identified
so that appropriate processing can be applied. For example, the user's language
preferences in a Web browser can be used to select Web pages appropriately. Language preferences can also be used to select among tools (such as
dictionaries) to assist in the processing or understanding of content in
different languages.

In addition, knowledge about the particular language used by some piece of information content might be
useful or even required by some types of processing; for example
spell-checking, computer-synthesized speech, Braille transcription, or
high-quality print renderings.

One means of indicating the language used is by labeling the information content
with an identifier or "tag". These
tags can be used to specify user preferences when selecting information content,
or for labeling additional attributes of content and associated resources.

Tags can also be used to indicate additional language attributes of content. For example, indicating specific information about the dialect,
writing system, or orthography used in a document or resource may enable the user to obtain information in a form
that they can understand, or important in processing or rendering the given content into an appropriate form or style.

This document specifies a particular identifier mechanism (the language tag) and a registration function for values
to be used to form tags. It also defines a mechanism for private use values and future extension.

2. The Language Tag

Language tags are used to help identify languages, whether spoken, written, signed, or otherwise signaled, for the purpose of communication. This includes constructed and artificial languages, but excludes languages not intended primarily for human communication, such as programming languages.

Each type of subtag is distinguished by
length, position in the tag, and content: subtags can be recognized solely by these features. This makes it possible
to construct a parser that can extract and assign some semantic information
to the subtags, even if the specific subtag values are not recognized. Thus a
parser need not have an up-to-date copy (or any copy at all) of the subtag registry to
perform most searching and matching operations.

The tags and their subtags, including private use and extensions, are to be
treated as case insensitive: there exist conventions for the
capitalization of some of the subtags, but these MUST NOT be taken to carry
meaning.

However, in the tags defined by this document, the uppercase US-ASCII letters in the range 'A' through 'Z' are considered equivalent and mapped directly to their US-ASCII lowercase equivalents in the range 'a' through 'z'. Thus the tag "mn-Cyrl-MN" is not
distinct from "MN-cYRL-mn" or "mN-cYrL-Mn" (or any other combination) and
each of these variations conveys the same meaning: Mongolian written in the
Cyrillic script as used in Mongolia.

Language tags are designed so that each subtag type has unique length and
content restrictions. These make
identification of the subtag's type possible, even if the content of
the subtag itself is unrecognized. This allows tags to be parsed and
processed without reference to the latest version of the underlying
standards or the IANA registry and makes the associated exception
handling when parsing tags simpler.

Subtags in the IANA registry that do not come from an underlying standard
can only appear in specific positions in a tag. Specifically, they can only
occur as primary language subtags or as variant subtags.

Note that sequences of private use and extension subtags MUST
occur at the end of the sequence of subtags and MUST NOT be
interspersed with subtags defined elsewhere in this document.

Single letter and digit subtags are reserved for current or future use.
These include the following current uses:

The single letter subtag 'x' is reserved to introduce a sequence of
private use subtags. The interpretation of any private use subtags is
defined solely by private agreement and is not defined by the rules
in this section or in any standard or registry defined in this document.

The primary language subtag is the first subtag in a language tag (with the exception of private use and certain grandfathered tags) and cannot be omitted.
The following rules apply to the primary language subtag:

The subtags in the range 'qaa' through 'qtz' are reserved for private use
in language tags. These subtags correspond to codes
reserved by ISO 639-2 for private use. These codes MAY be used for
non-registered primary-language subtags
(instead of using private use subtags following 'x-'). Please refer to
Section 4.5 (Considerations for Private Use Subtags) for more information on private use subtags.

All four character language subtags are reserved for possible future standardization.

All language subtags of 5 to 8 characters
in length in the IANA registry were defined via the registration process in
Section 3.4 (Registration Procedure for Subtags)
and MAY be used to form the primary language subtag.
At the time this document was created, there were no examples of
this kind of subtag and future registrations of this type will be
discouraged: primary languages are strongly RECOMMENDED for
registration with ISO 639 and proposals rejected by ISO 639/RA will be
closely scrutinized before they are registered with IANA.

The single character subtag 'x' as the primary subtag indicates that
the language tag consists solely of subtags whose meaning is defined by
private agreement. For example, in the tag "x-fr-CH", the subtags 'fr' and 'CH'
SHOULD NOT be taken to represent the French language or the country of Switzerland (or
any other value in the IANA registry) unless there is a private agreement in place
to do so. See Section 4.5 (Considerations for Private Use Subtags).

Other values MUST NOT be assigned to the primary subtag except by
revision or update of this document.

Note: For languages that have both an ISO 639-1 two character code
and an ISO 639-2 three character code, only the ISO 639-1 two character
code is defined in the IANA registry.

Note: For languages that have no ISO 639-1 two character code and
for which the ISO 639-2/T (Terminology) code and the ISO 639-2/B
(Bibliographic) codes differ, only the Terminology code is defined in
the IANA registry.
At the time this document was created, all languages that
had both kinds of three character
code were also assigned a two character code; it is not expected that
future assignments of this nature will occur.

"A language code already in ISO 639-2 at the point of freezing
ISO 639-1 shall not later be added to ISO 639-1. This is to
ensure consistency in usage over time, since users are directed
in Internet applications to employ the alpha-3 code when an
alpha-2 code for that language is not available."

In order to avoid instability in the canonical form of tags, if a
two character code is added to ISO 639-1 for a language for which a
three character code was already included in ISO 639-2, the two character code
MUST NOT be registered. See Section 3.3 (Stability of IANA Registry Entries).

For example, if some content were tagged with
'haw' (Hawaiian), which currently has no two character code, the tag
would not be invalidated if ISO 639-1 were to assign a two character code
to the Hawaiian language at a later date.

For example, one of the grandfathered IANA registrations
is "i-enochian". The subtag 'enochian' could be registered
in the IANA registry as a primary language subtag
(assuming that ISO 639 does not register this language first),
making tags such as "enochian-AQ" and "enochian-Latn" valid.

Three letter subtags immediately following the primary
subtag are reserved for future standardization, anticipating work that
is currently under way on ISO 639.

Extended language subtags MUST follow the primary subtag and precede
any other subtags.

There MAY be up to three extended language
subtags.

Extended language subtags MUST NOT be registered or used to form language tags. Their syntax is described here so that implementations can be compatible with any future revision of this document which does provide for their registration.

Extended language subtag records, once they appear in the registry, MUST include exactly one 'Prefix' field indicating an appropriate language subtag or sequence of subtags that MUST always appear as a prefix to the extended language subtag.

Example: In a future revision or update of this document, the tag
"zh-gan" (registered under RFC 3066) might become a valid
non-grandfathered (that is, redundant) tag in which the subtag 'gan'
might represent the Chinese dialect 'Gan'.

Script subtags MUST immediately follow
the primary language subtag and all extended language subtags and
MUST occur before any other type of subtag described below.

The script subtags 'Qaaa' through 'Qabx' are reserved for private use in
language tags. These
subtags correspond to codes reserved by ISO 15924 for private use.
These codes MAY be used for non-registered script values. Please refer to
Section 4.5 (Considerations for Private Use Subtags) for more information on private use subtags.

There MUST be at most one script subtag in a language tag and the script subtag SHOULD be omitted when it adds no distinguishing value to the tag or when the primary language subtag's record includes a Suppress-Script field listing the applicable script subtag.

Region subtags are used to indicate linguistic variations associated with or appropriate to a specific country, territory, or region. Typically, a region subtag is used to indicate regional dialects or usage, or region-specific spelling conventions. A region subtag can also be used to indicate that content is expressed in a way that is appropriate for use throughout a region; for instance, Spanish content tailored to be useful throughout Latin America.

The following rules apply to the region subtags:

Region subtags MUST follow any
language, extended language, or script subtags and MUST precede all
other subtags.

UN numeric codes assigned to 'macro-geographical
(continental)' or sub-regions MUST be registered in the registry. These codes are not associated with an assigned
ISO 3166 alpha-2 code and represent supra-national areas, usually covering more than one nation, state, province, or territory.

B

UN numeric codes for 'economic groupings' or 'other groupings' MUST NOT be
registered in the IANA registry and MUST NOT be used to form language tags.

C

UN numeric codes for countries or areas with ambiguous ISO 3166 alpha-2 codes, when entered into the registry, MUST be defined according to the rules in Section 3.3 (Stability of IANA Registry Entries) and MUST be used to form language tags that represent the country or region for which they are defined.

D

UN numeric codes for countries or areas for which there is an associated ISO 3166 alpha-2 code in the registry MUST NOT be entered into the registry and MUST NOT be used to form language tags. Note that the ISO 3166-based subtag in the registry MUST actually be associated with the UN M.49 code in question.

All other UN numeric codes for countries or areas which do not have an associated ISO 3166 alpha-2 code MUST NOT be entered into the registry and MUST NOT be used to form language tags. For more information about these codes, see Section 3.3 (Stability of IANA Registry Entries).

Note: The alphanumeric codes in Appendix X of the UN document MUST NOT be entered into the registry
and MUST NOT be used to form language tags. (At the time this document was created these values
match the ISO 3166 alpha-2 codes.)

There MUST be at most one region subtag in a language tag and the region subtag MAY be omitted, as when it adds no distinguishing value to the tag.

The region subtags 'AA', 'QM'-'QZ', 'XA'-'XZ', and 'ZZ'
are reserved for private use in language
tags. These subtags correspond to codes reserved by ISO 3166 for private
use. These codes
MAY be used for private use region subtags (instead of using a private use
subtag sequence). Please refer to
Section 4.5 (Considerations for Private Use Subtags) for more information on private use subtags.

"de-CH" represents German ('de') as used in Switzerland ('CH').

"sr-Latn-CS" represents Serbian ('sr') written using
Latin script ('Latn') as used in Serbia and Montenegro ('CS').

Variant subtags are used to indicate additional, well-recognized variations that define a language or its dialects which are not covered by other available subtags. The following rules apply to the variant subtags:

Variant subtags MUST follow all of the other defined subtags, but
precede any extension or private use subtag sequences.

More than one variant MAY be used to form the language tag.

Variant subtags MUST be registered with IANA according to the rules in
Section 3.4 (Registration Procedure for Subtags) of this document before being used to form
language tags. In order to distinguish variants from other types of subtags,
registrations MUST meet the following length and content restrictions:

Variant subtags that begin with a letter (a-z, A-Z) MUST be at
least five characters long.

Variant subtags that begin with a digit (0-9) MUST be at least
four characters long.

Variant subtag records in the language subtag registry MAY include one or more 'Prefix' fields, which indicates the language tag or tags that would make a suitable prefix (with other subtags, as appropriate) in forming a language tag with the variant. For example, the subtag 'nedis' has a Prefix of "sl", making it suitable to form language tags such as "sl-nedis" and "sl-IT-nedis", but not suitable for use in a tag such as "zh-nedis" or "it-IT-nedis".

"sl-nedis" represents the Natisone or Nadiza dialect of Slovenian.

"de-CH-1996" represents German as used in Switzerland
and as written using the spelling reform beginning in the year 1996 C.E.

Most variants that share a prefix are mutually exclusive. For example, the German orthographic variations '1996' and '1901' SHOULD NOT be used in the same tag, as they represent the dates of different spelling reforms. A variant that can meaningfully be used in combination with another variant SHOULD include a 'Prefix' field in its registry record that lists that other variant. For example, if another German variant 'example' were created that made sense to use with '1996', then 'example' should include two Prefix fields: "de" and "de-1996".

Extension subtags are separated from the other subtags defined in
this document by a single-letter subtag ("singleton"). The singleton
MUST be one allocated to a registration authority via the
mechanism described
in Section 3.6 (Extensions and Extensions Namespace) and MUST NOT be the letter 'x', which
is reserved for private use subtag sequences.

Note: Private use subtag sequences starting with the singleton
subtag 'x' are described below.

An extension MUST follow at least a primary language subtag. That is,
a language tag cannot begin with an extension. Extensions extend language
tags, they do not override or replace them. For example, "a-value" is not
a well-formed language tag, while "de-a-value" is.

Each singleton subtag MUST appear at most one time in each tag (other than
as a private use subtag). That is,
singleton subtags MUST NOT be repeated. For example, the tag
"en-a-bbb-a-ccc" is invalid because the subtag 'a' appears twice. Note that the tag "en-a-bbb-x-a-ccc" is valid because the second appearance of the singleton 'a' is in a private use sequence.

Extension subtags MUST meet all of the requirements for the content
and format of subtags defined in this document.

Extension subtags MUST meet whatever requirements are set by the
document that defines their singleton prefix and whatever requirements are
provided by the maintaining authority.

Each extension subtag MUST be from two to eight characters long
and consist solely of letters or digits, with each subtag separated by
a single '-'.

Each singleton MUST be followed by at least one extension subtag.
For example, the tag "tlh-a-b-foo" is invalid because the first singleton
'a' is followed immediately by another singleton 'b'.

Extension subtags MUST follow all language, extended language, script,
region and variant subtags in a tag.

All subtags following the singleton and before another singleton
are part of the extension. Example: In the tag "fr-a-Latn", the subtag
'Latn' does not represent the script subtag 'Latn' defined
in the IANA Language Subtag Registry. Its meaning is defined
by the extension 'a'.

Private use subtags are used to indicate distinctions in language important in a given context by private agreement. The following rules apply to private use subtags:

Private use subtags are separated from the other subtags
defined in this document by the reserved single-character subtag 'x'.

Private use subtags MUST conform to the format and content constraints defined in the ABNF for all subtags.

Private use subtags MUST follow all language, extended language,
script, region, variant, and extension subtags in the tag. Another
way of saying this is that all subtags following the singleton 'x'
MUST be considered private use. Example: The subtag 'US' in the
tag "en-x-US" is a private use subtag.

A tag MAY consist entirely of private use
subtags.

No source is defined for private use subtags. Use of private use
subtags is by private agreement only.

For example: Users who wished to utilize codes from the Ethnologue publication of SIL International for
language identification might agree to exchange tags such as
"az-Arab-x-AZE-derbend". This example contains two private use subtags.
The first is 'AZE' and the second is 'derbend'.

Existing IANA-registered language tags from RFC 1766 and/or RFC 3066 maintain their validity. These tags will be maintained in the registry in records of either the "grandfathered"
or "redundant" type. For
more information see Section 3.7 (Initialization of the Registry).

It is important to note that all language tags formed under the guidelines in this document were either legal, well-formed tags or could have been registered under RFC 3066.

Implementations sometimes need to describe their capabilities with regard to the rules and practices described in this document. There are two classes
of conforming implementations described by this document: "well-formed" processors and "validating"
processors. Claims of conformance SHOULD explicitly reference one of these
definitions.

An implementation that claims to check for well-formed language tags MUST:

Check that the tag and all of its subtags, including extension
and private use subtags, conform to the ABNF or that the tag is on
the list of grandfathered tags.

Check that singleton subtags that identify extensions do not repeat.
For example, the tag "en-a-xx-b-yy-a-zz" is not well-formed.

Specify the particular registry date for which the implementation performs
validation of subtags.

Check that either the tag is a grandfathered tag, or that all language,
script, region, and variant subtags consist of valid codes for use in
language tags according to the IANA registry as of the particular date
specified by the implementation.

For any such extensions supported, check that all subtags used in
that extension are valid.

For variant and extended language subtags, if the registry contains one or more 'Prefix' fields for that subtag, check that the tag matches at least one prefix. The tag matches if all the subtags in the 'Prefix' also appear in the tag. For example, the prefix "es-CO" matches the tag "es-Latn-CO-x-private" because both the 'es' language subtag and 'CO' region subtag appear in the tag.

3. Registry Format and Maintenance

This section defines the Language Subtag Registry and the maintenance and update procedures associated with it.

The language subtag registry will be maintained so that, except for
extension subtags, it is possible to validate all of the subtags that appear in
a language tag under the provisions of this document or
its revisions or successors. In addition, the meaning of the various subtags
will be unambiguous and stable over time. (The meaning of private use subtags,
of course, is not defined by the IANA registry.)

The registry defined under this document
contains a comprehensive list of all of the subtags valid in language tags. This
allows implementers a straightforward and reliable way to validate language tags.

The IANA Language Subtag Registry ("the registry") consists of a text file that is
machine readable in the format described in this section, plus copies of the
registration forms approved by the Language Subtag Reviewer in accordance with
the process described in Section 3.4 (Registration Procedure for Subtags). With the exception
of the registration forms for grandfathered and redundant tags, no registration records
will be maintained for the initial set of subtags.

The sequence '..' (%x2E.2E) in a field-body denotes a range of values. Such a range represents all
subtags of the same length that are alphabetically within that range,
including the values explicitly mentioned. For example 'a..c' denotes the
values 'a', 'b', and 'c'.

The first record in the file contains the single field whose field-name is "File-Date". The field-body of this record contains the last
modification date of this copy of the registry, making it possible to compare different versions of the registry. The registry on the IANA website is
the most current. Versions with an older date than that one
are not up-to-date.

File-Date: 2004-06-28
%%

Subsequent records represent subtags in the registry. Each of the fields in each
record
MUST occur no more than once, unless otherwise noted below.
Each record MUST contain the following fields:

'Type'

Type's field-value MUST consist of one of the following strings: "language",
"extlang", "script", "region", "variant", "grandfathered", and "redundant"
and denotes the type of tag or subtag.

Either 'Subtag' or 'Tag'

Subtag's field-value contains the subtag being defined. This field MUST only appear in records of whose 'Type' has one of these values: "language", "extlang", "script", "region", or "variant".

Tag's field-value contains a complete language tag. This field MUST only appear in records whose 'Type' has one of these values: "grandfathered" or "redundant".

Description

Description's field-value contains a non-normative description of the subtag
or tag.

Added

Added's field-value contains the date the record was added to the registry.

The 'Subtag' or 'Tag' field MUST use lowercase letters to form the subtag or tag, with two exceptions. Subtags whose 'Type' field is 'script' (in other words, subtags defined by ISO 15924) MUST use titlecase. Subtags whose 'Type' field is 'region' (in other words, subtags defined by ISO 3166) MUST use uppercase. These exceptions mirror the use of case in the underlying standards.

The field 'Description' MAY appear more than one time. At least one of the 'Description' fields MUST contain a description of the tag being registered written or transcribed into the Latin script; the same or additional fields MAY also include a description in a non-Latin script. The 'Description' field is used for identification purposes and SHOULD NOT be taken to represent the actual native name of the language or variation or to be in any particular language. Most descriptions are taken directly from source standards such as ISO 639 or ISO 3166.

Note: Descriptions in
registry entries that correspond to ISO 639, ISO 15924, ISO 3166 or UN M.49 codes are
intended only to indicate the meaning of that identifier as defined in
the source standard at the time it was added to the registry. The description does not replace the content of the source
standard itself.
The descriptions are not intended to be the English localized names
for the subtags. Localization or translation of language tag and subtag descriptions is out of scope
of this document.

Each record MAY also contain the following fields:

Preferred-Value

For fields of type 'language', 'extlang', 'script', 'region', and 'variant', 'Preferred-Value' contains a subtag of the same 'Type' which is preferred for forming the language tag.

For fields of type 'grandfathered' and 'redundant', a canonical mapping to a complete language tag.

Deprecated

Deprecated's field-value contains the date the record was deprecated.

Prefix

Prefix's field-value contains a language tag with which this subtag MAY be used to form a new language tag,
perhaps with other subtags as well. This field MUST only appear in records whose 'Type' field-value is 'variant' or 'extlang'. For example, the 'Prefix' for the variant 'nedis' is 'sl', meaning that the tags "sl-nedis" and "sl-IT-nedis" might be appropriate while the tag "is-nedis" is not.

Comments

Comments contains additional information about the subtag, as
deemed appropriate for understanding the registry and implementing language
tags using the subtag or tag.

Suppress-Script

Suppress-Script contains a script subtag that SHOULD NOT be used to form language tags with the associated primary language subtag. This field MUST only appear in records whose 'Type' field-value is 'language'. See Section 4.1 (Choice of Language Tag).

The field 'Deprecated' MAY be added to any record via the maintenance process described in Section 3.2 (Maintenance of the Registry) or via the registration process described in Section 3.4 (Registration Procedure for Subtags). Usually the addition of a 'Deprecated' field is due to the action of one of the standards bodies, such as ISO 3166, withdrawing a code. In some historical cases it might not have been possible to reconstruct the original deprecation date. For these cases, an approximate date appears in the registry. Although valid in language tags, subtags and tags with a 'Deprecated' field are deprecated and validating processors SHOULD NOT generate these subtags. Note that a record that contains a 'Deprecated' field and no corresponding 'Preferred-Value' field has no replacement mapping.

The field 'Preferred-Value' contains a mapping between the record in which it appears and a tag or subtag which SHOULD be preferred when selected language tags. These values form three groups:

ISO 639 language codes which were later withdrawn in favor of other codes. These values are mostly a historical curiosity.

ISO 3166 region codes which have been withdrawn in favor of a new code. This sometimes happens when a country changes its name or administration in such a way that warrants a new region code.

Tags grandfathered from RFC 3066. In many cases these tags have become obsolete because the values they represent were later encoded by ISO 639.

Records that contain a 'Preferred-Value' field MUST also have a 'Deprecated' field. This field contains a date of deprecation. Thus a language tag processor can use the registry to construct the valid, non-deprecated set of subtags for a given date. In addition, for any given tag, a processor can construct the set of valid language tags that correspond to that tag for all dates up to the date of the registry. The ability to do these mappings MAY be beneficial to applications that are matching, selecting, for filtering content based on its language tags.

Note that 'Preferred-Value' mappings in records of type 'region' sometimes do not represent exactly the same meaning as the original value. There are many reasons for a country code to be changed and the effect this has on the formation of language tags will depend on the nature of the change in question.

In particular, the 'Preferred-Value' field does not imply retagging content that uses the affected subtag.

The field 'Preferred-Value' MUST NOT be modified once created in the registry. The field MAY be added to records of type "grandfathered" and "region" according to the rules in Section 3.2 (Maintenance of the Registry). Otherwise the field MUST NOT be added to any record already in the registry.

The 'Preferred-Value' field in records of type "grandfathered" and "redundant" contains whole language tags that are strongly RECOMMENDED for use in place of the record's value. In many cases the mappings were created by deprecation of the tags during the period before this document was adopted. For example, the tag "no-nyn" was deprecated in favor of the ISO 639-1 defined language code 'nn'.

Records of type 'variant' MAY have more than one field of type 'Prefix'. Additional
fields of this type MAY be added to a 'variant' record via the registration process.

Records of type 'extlang' MUST have exactly one 'Prefix' field.

The field-value of the 'Prefix' field consists of a language tag whose subtags are appropriate to use with this subtag. For example, the variant subtag '1996' has a Prefix field of "de". This means that tags starting with the sequence "de-" are appropriate with this subtag, so "de-Latg-1996" and "de-CH-1996" are both acceptable, while the tag "fr-1996" is an inappropriate choice.

The field of type 'Prefix' MUST NOT be removed from any record. The field-value for this type of field MUST NOT be modified.

The field 'Comments' MAY appear more than once per record. This field MAY be inserted or changed via the registration
process and no guarantee of stability is provided. The content of this field is not restricted, except by the need to register the information, the suitability of the request, and by reasonable practical size limitations. Long texts about a particular subtag are frowned upon.

The field 'Suppress-Script' MUST only appear in records whose 'Type' field-value is 'language'. This field MUST NOT appear more than one time in a record. This field indicates a script used to write the overwhelming majority of documents for the given language and which therefore adds no distinguishing information to a language tag. It helps ensure greater compatibility between the language tags generated according to the rules in this document and language tags and tag processors or consumers based on RFC 3066. For example, virtually all Icelandic documents are written in the Latin script, making the subtag 'Latn' redundant in the tag "is-Latn".

Maintenance of the registry requires that as codes are assigned or withdrawn by
ISO 639, ISO 15924, ISO 3166, and UN M.49, the Language Subtag Reviewer MUST evaluate each
change, determine whether it conflicts with existing registry entries, and
submit the information to IANA for inclusion in the registry. If an change takes place and the Language Subtag Reviewer does not do this in a timely manner, then any interested party MAY use the procedure in Section 3.4 (Registration Procedure for Subtags) to register the appropriate update.

The set of redundant and grandfathered tags is permanent and stable: new entries in this section MUST NOT be added and existing entries MUST NOT be removed. Records of type 'grandfathered' MAY have their type converted to 'redundant': see Section 3.7 (Initialization of the Registry) for more information.

RFC 3066 tags that were deprecated prior to the adoption of this document are part of the list of grandfathered tags and their component subtags were not included as registered variants (although they remain eligible for registration). For example, the tag "art-lojban" was deprecated in favor of the language subtag 'jbo'.

The Language Subtag Reviewer MUST ensure that new subtags meet the requirements
in Section 4.1 (Choice of Language Tag) or submit an appropriate alternate subtag as described
in that section. When either a change or addition to the registry is needed, the Language Subtag Reviewer MUST prepare the complete record, including all fields, and forward it to IANA for insertion into the registry.

If record represents a new subtag that does not currently exist in the registry, then the message's subject line MUST include the word "INSERT". If the record represents a change to an existing subtag, then the subject line of the message MUST include the word "MODIFY". The message MUST contain both the record for the subtag being inserted or modified and the new File-Date record. Here is an example of what the body of the message might contain:

The stability of entries and their meaning in the registry is critical to the long term stability of language tags. The rules in this section guarantee that a specific language tag's meaning is stable over time and will not change.

These rules specifically deal with how changes to codes (including withdrawal and deprecation of codes) maintained by ISO 639, ISO 15924, ISO 3166, and UN M.49 are reflected in the IANA Language Subtag Registry. Assignments to the IANA Language Subtag Registry MUST follow the following stability rules:

Values in the fields 'Type', 'Subtag', 'Tag', 'Added', 'Deprecated' and 'Preferred-Value' MUST
NOT be changed and are guaranteed to be stable over time.

Values in the 'Description' field MUST NOT be changed in a way that would invalidate previously-existing tags. They MAY be broadened somewhat in scope, changed to add information, or adapted to the most common modern usage. For example, countries occasionally change their official names: an historical example of this would be "Upper Volta" changing to "Burkina Faso".

Values in the field 'Prefix' MAY be added to records of type 'variant' via the registration process.

Values in the field 'Prefix' MAY be modified, so long as the modifications broaden the set of prefixes. That is, a prefix MAY be replaced by one of its own prefixes. For example, the prefix "en-US" could be replaced by "en", but not by the prefixes "en-Latn", "fr", or "en-US-boont". If one of those prefixes were needed, a new Prefix SHOULD be registered.

Values in the field 'Prefix' MUST NOT be removed.

The field 'Comments' MAY be added, changed, modified, or removed via the registration process or any of the processes or considerations described in this section.

The field 'Suppress-Script' MAY be added or removed via the registration process.

Codes assigned by ISO 639, ISO 15924, and ISO 3166 that do not conflict with existing subtags of the associated type and whose meaning is not the same as an existing subtag of the same type are entered into the IANA registry as new records.

Codes assigned by ISO 639, ISO 15924, or ISO 3166 that are withdrawn by their respective maintenance or registration authority remain valid in language tags. A 'Deprecated' field containing the date of withdrawal is added to the record. If a new record of the same type is added that represents a replacement value, then a 'Preferred-Value' field MAY also be added. The registration process MAY be used to add comments about the withdrawal of the code by the respective standard.

Example

The region code 'TL' was assigned to the country 'Timor-Leste', replacing the code 'TP' (which was assigned to 'East Timor' when it was under administration by Portugal). The subtag 'TP' remains valid in language tags, but its record contains the a 'Preferred-Value' of 'TL' and its field 'Deprecated' contains the date the new code was assigned ('2004-07-06').

Codes assigned by ISO 639, ISO 15924, or ISO 3166 that conflict with existing subtags of the associated type, including subtags that are deprecated, MUST NOT be entered into the registry. The following additional considerations apply to subtag values that are reassigned:

A

For ISO 639 codes, if the newly assigned code's meaning is not represented by a subtag in the IANA registry, the Language Subtag Reviewer, as described in Section 3.4 (Registration Procedure for Subtags), SHALL prepare a proposal for entering in the IANA registry as soon as practical a registered language subtag as an alternate value for the new code. The form of the registered language subtag will be at the discretion of the Language Subtag Reviewer and MUST conform to other restrictions on language subtags in this document.

B

For all subtags whose meaning is derived from an external standard (i.e. ISO 639, ISO 15924, ISO 3166, or UN M.49), if a new meaning is assigned to an existing code and the new meaning broadens the meaning of that code, then the meaning for the associated subtag MAY be changed to match. The meaning of
a subtag MUST NOT be narrowed, however, as this can result in an
unknown proportion of the existing uses of a subtag becoming invalid. Note: ISO 639 MA/RA has adopted a similar stability policy.

C

For ISO 15924 codes, if the newly assigned code's meaning is not represented by a subtag in the IANA registry, the Language Subtag Reviewer, as described in Section 3.4 (Registration Procedure for Subtags), SHALL prepare a proposal for entering in the IANA registry as soon as practical a registered variant subtag as an alternate value for the new code. The form of the registered variant subtag will be at the discretion of the Language Subtag Reviewer and MUST conform to other restrictions on variant subtags in this document.

D

For ISO 3166 codes, if the newly assigned code's meaning is associated with the same UN M.49 code as another 'region' subtag, then the existing region subtag remains as the preferred value for that region and no new entry is created. A comment MAY be added to the existing region subtag indicating the relationship to the new ISO 3166 code.

E

For ISO 3166 codes, if the newly assigned code's meaning is associated with a UN M.49 code that is not represented by an existing region subtag, then the Language Subtag Reviewer, as described in Section 3.4 (Registration Procedure for Subtags), SHALL prepare a proposal for entering the appropriate UN M.49 country code as an entry in the IANA registry.

F

For ISO 3166 codes, if there is no associated UN numeric code, then the Language Subtag Reviewer SHALL petition the UN to create one. If there is no response from the UN within ninety days of the request being sent, the Language Subtag Reviewer SHALL prepare a proposal for entering in the IANA registry as soon as practical a registered variant subtag as an alternate value for the new code. The form of the registered variant subtag will be at the discretion of the Language Subtag Reviewer and MUST conform to other restrictions on variant subtags in this document. This situation is very unlikely to ever occur.

UN M.49 has codes for both countries and areas (such as '276' for Germany) and geographical regions and sub-regions (such as '150' for Europe). UN M.49 country or area codes for which there is no corresponding ISO 3166 code SHOULD NOT be registered, except as a surrogate for an ISO 3166 code that is blocked from registration by an existing subtag. If such a code becomes necessary, then the registration authority for ISO 3166 SHOULD first be petitioned to assign a code to the region. If the petition for a code assignment by ISO 3166 is refused
or not acted on in a timely manner, the registration process described in Section 3.4 (Registration Procedure for Subtags) MAY then be used to register the corresponding UN M.49 code. At the time this document was written, there were only four such codes: 830 (Channel Islands), 831 (Guernsey), 832 (Jersey), and 833 (Isle of Man). This way UN M.49 codes remain available as the value of last resort in cases where ISO 3166 reassigns a deprecated value in the registry.

Stability provisions apply to grandfathered tags with this exception:
should all of
the subtags in a grandfathered tag become valid subtags in the IANA registry,
then the field 'Type' in that record is changed from 'grandfathered' to 'redundant'. Note that this
will not affect
language tags that match the grandfathered tag, since these tags will now
match valid generative subtag sequences. For example, if the subtag 'gan'
in the language tag "zh-gan" were to be registered as an extended language
subtag, then the grandfathered tag "zh-gan" would be deprecated (but
existing content or implementations that use "zh-gan" would remain valid).

This procedure MAY also be used to register or alter the information for the "Description", "Comments", "Deprecated", or "Prefix" fields in a subtag's record as described in Section 3.3 (Stability of IANA Registry Entries). Changes to all other fields in the IANA
registry are NOT permitted.

Registering a new subtag or requesting modifications to an existing tag or subtag starts with the requester filling out
the registration form reproduced below. Note that each response is not limited
in size so that the request can adequately describe the registration.
The fields in the "Record Requested" section SHOULD follow the requirements in Section 3.1 (Format of the IANA Language Subtag Registry).

LANGUAGE SUBTAG REGISTRATION FORM
1. Name of requester:
2. E-mail address of requester:
3. Record Requested:
Type:
Subtag:
Description:
Prefix:
Preferred-Value:
Deprecated:
Suppress-Script:
Comments:
4. Intended meaning of the subtag:
5. Reference to published description
of the language (book or article):
6. Any other relevant information:

Figure 5

The subtag registration form MUST be sent to <ietf-languages@iana.org> for
a two week review period before it can be submitted to IANA. (This is an open list and can be joined by sending a request to <ietf-languages-request@iana.org>.)

Variant and extlang subtags are always registered for use with a particular range of language tags. For example, the subtag 'rozaj' is
intended for use with language tags that start with the primary language subtag "sl", since
Resian is a dialect of Slovenian. Thus the subtag 'rozaj' could be included in tags such as "sl-Latn-rozaj" or
"sl-IT-rozaj". This information is stored in the "Prefix" field in the registry. Variant registration requests are REQUIRED to include at least one "Prefix" field in the registration form.

The 'Prefix' field for a given registered subtag exists
in the IANA registry as a guide to usage. Additional prefixes MAY be added by filing an additional registration form. In that form, the "Any other
relevant information:" field MUST indicate that it is the addition of
a prefix.

Requests to add a prefix to a
variant subtag that imply a different semantic meaning will probably be rejected.
For example,
a request to add the prefix "de" to the subtag 'nedis' so that
the tag "de-nedis" represented some German dialect would be rejected. The 'nedis'
subtag represents a particular Slovenian dialect and the additional registration
would change the semantic meaning assigned to the subtag. A separate subtag SHOULD
be proposed instead.

The 'Description' field MUST contain a description of the tag being registered written or transcribed into the Latin script; it MAY also include a description in a non-Latin script. Non-ASCII characters MUST be escaped using the syntax described in Section 3.1 (Format of the IANA Language Subtag Registry). The 'Description' field is used for identification purposes and doesn't necessarily represent the actual native name of the language or variation or to be in any particular language.

While the 'Description' field itself is not guaranteed to be stable and errata corrections MAY be undertaken from time to time, attempts to provide translations or transcriptions of entries in the registry itself will probably be frowned upon by the community or rejected outright, as changes of this nature have an impact on the provisions in Section 3.3 (Stability of IANA Registry Entries).

The Language Subtag Reviewer is responsible for responding to requests for the registration of subtags through the registration process and is appointed by the IESG.

When the two week period has passed the Language Subtag Reviewer either forwards the record to be inserted or modified to iana@iana.org according to the procedure described in Section 3.2 (Maintenance of the Registry), or
rejects the request because of significant objections raised on the list or due to
problems with constraints in this document (which MUST be explicitly cited).
The reviewer MAY also extend the review period in two week increments to permit further discussion. The reviewer MUST indicate on the list whether the registration has been accepted, rejected, or extended following each two week period.

Note that the
reviewer MAY raise objections on the list if he or she so desires. The
important thing is that the objection MUST be made publicly.

The applicant is free to modify a rejected application with additional
information and submit it again; this restarts the two week comment period.

Updates or changes to existing records follow the same procedure
as new registrations. The Language Subtag Reviewer
decides whether there is consensus to update the
registration following the two week review period;
normally objections by the original registrant will
carry extra weight in forming such a consensus.

Registrations are permanent and stable. Once registered, subtags will not be
removed from the registry and will remain a valid way in which to specify
a specific language or variant.

Note: The purpose of the "Description" in the
registration form is intended as an aid to people
trying to verify whether a language is registered or what language or
language variation a particular
subtag refers to. In most cases, reference to an authoritative grammar or
dictionary of that language will be useful; in cases where no such work
exists, other well known works describing that language or in that language
MAY be appropriate. The subtag reviewer decides what constitutes "good
enough" reference material. This requirement is not intended to exclude
particular languages or dialects due to the size of the speaker
population or lack of a standardized orthography. Minority languages will be
considered equally on their own merits.

Possibilities for registration of subtags or information about subtags include:

Primary language subtags for languages not listed in ISO 639 that are
not variants of any listed
or registered language MAY be registered. At the time this document was
created there were no examples of this form of subtag. Before
attempting to register a language subtag, there MUST be an
attempt to register the language with ISO 639. Subtags MUST NOT be registered for codes that exist in ISO 639-1 or ISO 639-2, which are
under consideration by the ISO 639 maintenance or registration
authorities, or which have never been attempted for registration with those
authorities. If ISO 639 has previously rejected a language for registration,
it is reasonable to assume that there must be additional very compelling evidence
of need before it will be registered in the IANA registry (to the extent that
it is very unlikely that any subtags will be registered of this type).

Dialect or other divisions or variations within a language, its orthography, writing system, regional or historical usage, transliteration or other transformation, or distinguishing variation MAY be registered as variant subtags. An example is the 'rozaj' subtag (the Resian dialect of Slovenian).

Extension subtags are those introduced by single-letter subtags other than
'x'. They are reserved for the generation of identifiers which contain
a language component, and are compatible with applications that understand language tags.

The structure and form of extensions are defined by this document so that
implementations can be created that are forward compatible with applications
that might be created using single-letter subtags in the future. In addition,
defining a mechanism for maintaining single-letter subtags will lend
stability to this document by reducing the likely need for future revisions
or updates.

The specification MUST reference the specific version or revision
of this document that governs its creation and MUST reference
this section of this document.

The specification and all subtags defined by the specification
MUST follow the ABNF and other rules for the formation of tags and
subtags as defined in this document. In particular it MUST specify
that case is not significant and that subtags MUST NOT exceed eight characters in length.

The specification MUST specify a canonical representation.

The specification of valid subtags MUST be available
over the Internet and at no cost.

The specification MUST be in the public domain or available
via a royalty-free license acceptable to the IETF and specified
in the RFC.

The specification MUST be versioned and each version of the
specification MUST be numbered, dated, and stable.

The specification MUST be stable. That is, extension subtags,
once defined by a specification, MUST NOT be retracted or change in
meaning in any substantial way.

The specification MUST include in a separate section the registration form reproduced in this section (below) to be used in registering the extension upon publication as an RFC.

IANA MUST be informed of changes to the contact information and
URL for the specification.

IANA will maintain a registry of allocated single-letter (singleton) subtags. This registry MUST use the record-jar format described by the ABNF in Section 3.1 (Format of the IANA Language Subtag Registry). Upon publication of an extension as an RFC, the maintaining authority defined in the RFC MUST forward this registration form to iesg@ietf.org, who MUST forward the request to iana@iana.org. The maintaining authority of the extension MUST maintain the accuracy of the record by sending an updated full copy of the record to iana@iana.org with the subject line "LANGUAGE TAG EXTENSION UPDATE" whenever content changes. Only the 'Comments', 'Contact_Email', 'Mailing_List', and 'URL' fields MAY be modified in these updates.

'Identifier' contains the single letter subtag (singleton) assigned to the extension. The Internet-Draft submitted to define the extension SHOULD specify which letter to use, although the IESG MAY change the assignment when approving the RFC.

'Description' contains the name and description of the extension.

'Comments' is an OPTIONAL field and MAY contain a broader description of the extension.

'Authority' contains the name of the maintaining authority for the extension.

'Contact_Email' contains the email address used to contact the maintaining authority.

'Mailing_List' contains the URL or subscription email address of the mailing list used by the maintaining authority.

'URL' contains the URL of the registry for this extension.

The determination of whether an Internet-Draft meets the above conditions
and the decision to grant
or withhold such authority rests solely with the IESG, and is subject to the
normal review and appeals process associated with the RFC process.

Extension authors are strongly cautioned that
many (including most well-formed) processors will be unaware of any special
relationships or meaning inherent in the order
of extension subtags. Extension authors SHOULD avoid subtag relationships or canonicalization mechanisms that interfere
with matching or
with length restrictions that sometimes exist in common protocols where the extension is used.
In particular, applications MAY truncate the
subtags in doing matching or in fitting into limited lengths, so it is
RECOMMENDED that the most significant information be in the most significant (left-most) subtags,
and that the specification gracefully handle truncated subtags.

When a language tag is to be used in a specific, known, protocol, it is RECOMMENDED that that the language tag not contain extensions not supported by that protocol. In addition, note that some protocols MAY impose upper limits on the length of the strings used to store or transport the language tag.

4. Formation and Processing of Language Tags

One is sometimes faced with the choice between several possible tags for the same body
of text.

Interoperability is best served when all users use the same language tag
in order to represent the same language. If an application has
requirements that make the rules here inapplicable, then that application
risks damaging interoperability. It is strongly RECOMMENDED that users not define their own rules for language tag choice.

Subtags SHOULD only be used where they add useful distinguishing information; extraneous subtags interfere with the meaning, understanding, and processing of language tags. In particular, users and implementations SHOULD follow the 'Prefix' and 'Suppress-Script' fields in the registry (defined in Section 3.1 (Format of the IANA Language Subtag Registry)): these fields provide guidance on when specific additional subtags SHOULD (and SHOULD NOT) be used in a language tag.

Of particular note, many applications can benefit from the use of script subtags in language tags, as long as the use is consistent for a given context. Script subtags were not formally defined in RFC 3066 and their use can affect matching and subtag identification by implementations of RFC 3066, as these subtags appear between the primary language and region subtags. For example, if a user requests content in an implementation of Section 2.5 of [RFC3066] (Alvestrand, H., “Tags for the Identification of Languages,” January 2001.) using the language range "en-US", content labeled "en-Latn-US" will not match the request. Therefore it is important to know when script subtags will customarily be used and when they ought not be used. In the registry, the Suppress-Script field helps ensure greater compatibility between the language tags generated according to the rules in this document and language tags and tag processors or consumers based on RFC 3066 by defining when users SHOULD NOT include a script subtag with a particular primary language subtag.

Extended language subtags (type 'extlang' in the registry, see Section 3.1 (Format of the IANA Language Subtag Registry)) also appear between the primary language and region subtags and are reserved for future standardization. Applications might benefit from their judicious use in forming language tags in the future. Similar recommendations are expected to apply to their use as apply to script subtags.

Standards, protocols and applications that reference this document normatively
but apply different rules to the ones given in this section MUST specify
how the procedure varies from the one given here.

The choice of subtags used to form a language tag SHOULD be guided by the following rules:

Use as precise a tag as possible, but no more specific than is justified.
Avoid using subtags that are not important for distinguishing content in an application.

For example, 'de' might suffice for tagging an email written in German,
while "de-CH-1996" is probably unnecessarily precise for such a task.

The script subtag SHOULD NOT be used to form language tags unless the script adds some distinguishing information to the tag. The field 'Suppress-Script' in the primary language record in the registry indicates which script subtags do not add distinguishing information for most applications.

For example, the subtag 'Latn' should not be used with the primary language 'en' because nearly all English documents are written in the Latin script and it adds no distinguishing information. However, if a document were written in English mixing Latin script with another script such as Braille ('Brai'), then it might be appropriate to choose to indicate both scripts to aid in content selection, such as the application of a style sheet.

If a tag or subtag has a 'Preferred-Value' field in its registry entry, then the value of that field SHOULD be used to form the language tag in preference to the tag or subtag in which the preferred value appears.

For example, use 'he' for Hebrew in preference
to 'iw'.

The 'und' (Undetermined) primary language subtag SHOULD NOT be used
to label content, even if the language is unknown. Omitting the
language tag altogether is preferred to using a tag with a primary
language subtag of 'und'. The 'und' subtag MAY be useful for protocols
that require a language tag to be provided. The 'und' subtag MAY also
be useful when matching language tags in certain situations.

The 'mul' (Multiple) primary language subtag SHOULD NOT be used whenever the protocol allows the separate tags for multiple languages, as is the case for the Content-Language header in HTTP. The 'mul' subtag conveys little useful information: content in multiple
languages SHOULD individually tag the languages where they appear or
otherwise indicate the actual language in preference to the 'mul' subtag.

The same variant subtag SHOULD NOT be used more than once within a language tag.

For example, do not use "de-DE-1901-1901".

To ensure consistent backward compatibility, this document contains several
provisions to account for potential instability in the standards used to define
the subtags that make up language tags. These provisions
mean that no language tag created under the rules in this document will become
obsolete.

The relationship between the tag and the information it relates to is defined by
the context in which the tag appears. Accordingly, this section
gives only possible examples of its usage.

For a single information object, the associated language tags might be
interpreted as the set of languages that is necessary for a complete
comprehension of the complete object. Example: Plain text documents.

For an aggregation of information objects, the associated language tags
could be taken as the set of languages used inside components of that
aggregation. Examples: Document stores and libraries.

For information objects whose purpose is to provide alternatives, the
associated language tags could be regarded as a hint that the content is
provided in several languages, and that one has to inspect each of the
alternatives in order to find its language or languages. In this case, the
presence of multiple tags might not mean that one needs to be multi-lingual to
get complete understanding of the document. Example: MIME
multipart/alternative.

In markup languages, such as HTML and XML, language information can be
added to each part of the document identified by the markup structure
(including the whole document itself). For example, one could write
<span lang="fr">C'est la vie.</span> inside a Norwegian document;
the Norwegian-speaking user could then access a French-Norwegian dictionary
to find out what the marked section meant. If the user were listening to that
document through a speech synthesis interface, this formation could be used
to signal the synthesizer to appropriately apply French text-to-speech
pronunciation rules to that span of text, instead of applying the
inappropriate Norwegian rules.

Language tags are related when they contain a similar sequence of subtags. For example, if a language tag B contains language tag A as a prefix, then B is typically
"narrower" or "more specific" than A. Thus "zh-Hant-TW" is more specific
than "zh-Hant".

This relationship is not guaranteed in all cases: specifically,
languages that begin with the same sequence of subtags are NOT guaranteed to be
mutually intelligible, although they might be. For example, the tag "az"
shares a prefix with both "az-Latn" (Azerbaijani written using the Latin script)
and "az-Cyrl" (Azerbaijani written using the Cyrillic script). A person fluent
in one script might not be able to read the other, even though the text might be
identical. Content tagged as "az" most probably is written in just one script
and thus might not be intelligible to a reader familiar with the other script.

Neither the language tag syntax nor other requirements in this document impose a fixed upper limit on the number of subtags in a language tag (and thus an upper bound on the size of a tag). The language tag syntax suggests that, depending on the specific language, more subtags (and thus a longer tag) are sometimes necessary to completely identify the language for certain applications; thus it is possible to envision long or complex subtag sequences.

Some applications and protocols are forced to allocate fixed buffer sizes or otherwise limit the length of a language tag. A conformant implementation or specification MAY refuse to support the storage of
language tags
which exceed a specified length. Any such limitation SHOULD be clearly documented, and such documentation SHOULD include what happens to longer tags (for example, whether an error value is generated or the language tag is truncated). A protocol that allows tags to be truncated at an arbitrary limit, without giving any indication of what that limit is, has the potential for causing harm by changing the meaning of tags in substantial ways.

Implementations SHOULD NOT truncate language tags unless the meaning of the tag is purposefully being changed, or unless the tag does not fit into a limited buffer size specified by a protocol for storage or transmission.

Implementations SHOULD warn the user when a tag is truncated since truncation changes the semantic meaning of the tag.

Implementations of protocols or specifications that are space constrained but do not have a fixed limit SHOULD use the longest possible tag in preference to truncation.

Protocols or specifications that specify limited buffer sizes for language tags MUST allow for language tags of up to 33 characters.

Protocols or specifications that specify limited buffer sizes for language tags SHOULD allow for language tags of at least
42 characters.

The following illustration shows how the 42-character recommendation was derived. The combination of language and extended language subtags was chosen for future compatibility. At up to 15 characters, this combination is longer than the longest possible primary language subtag (8 characters):

Truncation of a language tag alters the meaning of the tag, and thus SHOULD be avoided. However, truncation of language tags is sometimes necessary due to limited buffer sizes. Such truncation MUST NOT permit a subtag to be chopped off in the middle or the formation of invalid tags (for example, one ending with the "-" character).

This means that applications or protocols which truncate tags MUST do so
by progressively removing subtags along with their preceding "-" from the right side of the language tag until the tag is short enough for the given buffer. If the resulting tag ends with a single-character subtag, that subtag and its preceding "-" MUST also be removed. For example:

Redundant or grandfathered tags that have a Preferred-Value mapping
in the IANA registry (see Section 3.1 (Format of the IANA Language Subtag Registry)) MUST
be replaced with their mapped value. These items are either
deprecated mappings created before the adoption of this document
(such as the mapping of "no-nyn" to "nn" or "i-klingon" to "tlh")
or are the result of later registrations or additions to this
document (for example, "zh-guoyu" might be mapped to a
language-extlang combination such as "zh-cmn" by some future
update of this document).

Other subtags that have a Preferred-Value mapping
in the IANA registry (see Section 3.1 (Format of the IANA Language Subtag Registry)) MUST be
replaced with their mapped value. These items consist entirely of
clerical corrections to ISO 639-1 in which the deprecated subtags
have been maintained for compatibility purposes.

If more than one extension subtag sequence exists, the extension
sequences are ordered into case-insensitive ASCII order by singleton
subtag.

Example: The language tag "en-A-aaa-B-ccc-bbb-x-xyz" is in canonical form,
while "en-B-ccc-bbb-A-aaa-X-xyz" is well-formed but not in canonical form.

Example: The language tag "en-BU" (English as used in Burma) is
not canonical because the 'BU' subtag has a canonical mapping to 'MM' (Myanmar), although the tag "en-BU" maintains its validity.

Canonicalization of language tags does not imply anything about the use of upper or lowercase letters when processing or comparing subtags (and as described in Section 2.1 (Syntax)). All comparisons MUST be performed in a case-insensitive manner.

When performing canonicalization of language tags, processors MAY regularize the case of the subtags (that is, this process is OPTIONAL), following the case used in the registry. Note that this corresponds to the following casing rules: uppercase all non-initial two-letter subtags; titlecase all non-initial four-letter subtags; lowercase everything else.

Note: Case folding of ASCII letters in certain locales, unless carefully handled, sometimes produces non-ASCII character values. The Unicode Character Database file "SpecialCasing.txt" defines the specific cases that are known to cause problems with this. In particular, the letter 'i' (U+0069) in Turkish and Azerbaijani is uppercased to U+0130 (LATIN CAPITAL LETTER I WITH DOT ABOVE). Implementers SHOULD specify a locale-neutral casing operation to ensure that case folding of subtags does not produce this value, which is illegal in language tags. For example, if one were to uppercase the region subtag 'in' using Turkish locale rules, the sequence U+0130 U+004E would result instead of the expected 'IN'.

Note: if the field 'Deprecated' appears in a registry record without an accompanying 'Preferred-Value' field, then that tag or subtag is deprecated without a replacement. Validating processors SHOULD NOT generate tags that include these values, although the values are canonical when they appear in a language tag.

An extension MUST define any relationships that exist between the
various subtags in the extension and thus MAY define an alternate
canonicalization scheme for the extension's subtags. Extensions MAY
define how the order of the
extension's subtags are interpreted. For example, an extension could
define that its subtags are in canonical order when the subtags are placed
into ASCII order: that is, "en-a-aaa-bbb-ccc" instead of
"en-a-ccc-bbb-aaa". Another
extension might define that the order of the subtags influences their
semantic meaning (so that "en-b-ccc-bbb-aaa" has a different value from
"en-b-aaa-bbb-ccc"). However, extension specifications SHOULD be designed so that they are tolerant of the typical processes described in Section 3.6 (Extensions and Extensions Namespace).

Private use subtags, like all other subtags, MUST conform to the format and content constraints in the ABNF. Private use
subtags have no meaning outside the private agreement between
the parties that intend to use or exchange language tags that employ them. The same subtags MAY be used with a different meaning under a separate private agreement. They SHOULD NOT be used where alternatives exist and SHOULD NOT be used in content or protocols
intended for general use.

Private use subtags are simply useless for
information exchange without prior arrangement. The value and
semantic meaning of private use tags and of the subtags used
within such a language tag are not defined by this document.

Subtags defined in the IANA registry as having a specific private use meaning convey more information that a purely private use tag prefixed by the singleton subtag 'x'. For applications this additional information MAY be useful.

For example, the region subtags 'AA', 'ZZ' and in the ranges 'QM'-'QZ' and 'XA'-'XZ' (derived from ISO 3166 private use codes) MAY be used to form a language tag. A tag such as "zh-Hans-XQ" conveys a great deal of public, interchangeable information about the language material (that it is Chinese in the simplified Chinese script and is suitable for some geographic region 'XQ'). While the precise geographic region is not known outside of private agreement, the tag conveys far more information than an opaque tag such as "x-someLang", which contains no information about the language subtag or script subtag outside of the private agreement.

However, in some cases content tagged with private use subtags MAY interact with other systems in a different and possibly unsuitable manner compared to tags that use opaque, privately defined subtags, so the choice of the best approach sometimes depends on the particular domain in question.

Future work on the Language Subtag Registry SHALL be limited to inserting or replacing whole records preformatted for IANA by the Language Subtag Reviewer as described in Section 3.2 (Maintenance of the Registry) of this document. This simplifies IANA's work by limiting it to placing the text in the appropriate location in the registry.

Each record MUST be sent to iana@iana.org with a subject line indicating whether the enclosed record is an insertion of a new record (indicated by the word "INSERT" in the subject line) or a replacement of an existing record (indicated by the word "MODIFY" in the subject line). Records MUST NOT be deleted from the registry. IANA MUST place any inserted or modified records into the appropriate section of the language subtag registry, grouping the records by their 'Type' field. Inserted records MAY be placed anywhere in the appropriate section; there is no guarantee of the order of the records beyond grouping them together by 'Type'. Modified records MUST overwrite the record they replace.

Included in any request to insert or modify records MUST be a new File-Date record. This record MUST be placed first in the registry. In the event that the File-Date record present in the registry has a later date then the record being inserted or modified, the existing record MUST be preserved.

The Language Tag Extensions registry will also be generated and sent to IANA as described in Section 3.6 (Extensions and Extensions Namespace). This registry can contain at most 35 records and thus changes to this registry are expected to be very infrequent.

Future work by IANA on the Language Tag Extensions Registry is limited to two cases. First, the IESG MAY request that new records be inserted into this registry from time to time. These requests MUST include the record to insert in the exact format described in Section 3.6 (Extensions and Extensions Namespace). In addition, there MAY be occasional requests from the maintaining authority for a specific extension to update the contact information or URLs in the record. These requests MUST include the complete, updated record. IANA is not responsible for validating the information provided, only that it is properly formatted. It should reasonably be seen to come from the maintaining authority named in the record present in the registry.

6. Security Considerations

Language tags used in content negotiation, like any other information exchanged on the Internet, might be a source of concern because they might be used to infer the nationality of the
sender, and thus identify potential targets for surveillance.

This is a special case of the general problem that anything sent is visible
to the receiving party and possibly to third parties as well. It is useful to be aware that such concerns can exist in
some cases.

The language tag associated with a particular information item is of no
consequence whatsoever in determining whether that content might
contain possible homographs. The fact that a text is tagged as being
in one language or using a particular script subtag provides no assurance whatsoever that it does not
contain characters from scripts other than the one(s) associated with or specified by
that language tag.

Since there is no limit to the number of variant, private use, and extension
subtags, and consequently no limit on the possible length of a tag,
implementations need to guard against buffer overflow attacks. See Section 4.3 (Length Considerations) for details on language tag truncation, which can occur as a
consequence of defenses against buffer overflow.

Although the specification of valid subtags for an
extension (see: Section 3.6 (Extensions and Extensions Namespace)) MUST be available over the Internet, implementations SHOULD NOT
mechanically depend on it being always accessible, to prevent
denial-of-service attacks.

7. Character Set Considerations

The syntax in this document requires that language tags use only
the characters A-Z, a-z, 0-9, and HYPHEN-MINUS, which are present in
most character sets, so the composition of language tags should not have any
character set issues.

Rendering of characters based on the content of a
language tag is not addressed in this memo. Historically, some languages have relied on the use of specific character sets or other information in order to infer how a specific character should be rendered (notably this applies to language and culture specific variations of Han ideographs as used in Japanese, Chinese, and Korean). When language tags are applied to spans of text, rendering engines sometimes use that information in deciding which font to use in the absence of other information, particularly where languages with distinct writing traditions use the same characters.

8. Changes from RFC 3066

The main goals for this revision of language tags were the following:

Compatibility. All RFC 3066 language tags (including those in the IANA registry) remain valid in this specification. The changes in this document represent additional constraints on language tags. That is, in no case is the syntax more permissive and processors based on the ABNF and other provisions of RFC 3066 (such as those described in [XMLSchema] (Biron, P., Ed. and A. Malhotra, Ed., “XML Schema Part 2: Datatypes Second Edition,” 10 2004.)) will be able to process the tags described by this document. In addition, this document defines language tags in such as way as to ensure future compatibility.

Stability. Because of changes in the past in the underlying ISO standards, a valid RFC 3066 language tag could become invalid or have its meaning change. This has the potential of invalidating content that may have an extensive shelf-life. In this specification, once a language tag is valid, it remains valid forever.

Validity. The structure of language tags defined by this document makes it possible to determine if a particular tag is well-formed without regard for the actual content or "meaning" of the tag as a whole. This is important because the registry grows and underlying standards change over time. In addition, it must be possible to determine if a tag is valid (or not) for a given point in time in order to provide reproducible, testable results. This process must not be error-prone; otherwise implementations might give different results. By having an authoritative registry with specific versioning information, the validity of language tags at any point in time can be precisely determined (instead of interpolating values from many separate sources).

Utility. It is sometimes important to be able to differentiate between written forms of a language -- for many implementations this is more important
than distinguishing between the spoken variants of a language. Languages are written in a wide variety of different scripts, so this document provides for the generative use of ISO 15924 script codes. Like the generative use of ISO language and country codes in RFC 3066, this allows combinations to be produced without resorting to the registration process. The addition of UN M.49 codes provides for the generation of language tags with regional scope, which is also required by some applications.

The recast of the registry from containing whole language tags to subtags is a key part of this. An important feature of RFC 3066 was that it allowed generative use of subtags. This allows people to meaningfully use generated tags, without the delays in registering whole tags or the need to register all of the combinations that might be useful.

The choice of placing the extended language and script subtags between the primary language and region subtags was widely debated. This design was chosen because the prevalent matching and content negotiation schemes rely on the subtags being arranged in order of increasing specificity. That is, the subtags that mark a greater barrier to mutual intelligibility appear left-most in a tag. For example, when selecting content written in Azerbaijani, the script (Arabic, Cyrillic, or Latin) represents a greater barrier to understanding than any regional variations (those associated with Azerbaijan or Iran, for example). Individuals who prefer documents in a particular script, but can deal with the minor regional differences, can therefore select appropriate content. Applications that do not deal with written content will continue to omit these subtags.

Extensibility. Because of the widespread use of language tags, it is disruptive to have periodic revisions of the core specification, even in the face of demonstrated need. The extension mechanism provides for a way for independent RFCs to define extensions to language tags. These extensions have a very constrained, well-defined structure that prevent extensions from interfering with implementations of language tags defined in this document.

The document also anticipates features of ISO 639-3 with the addition of the extended language subtags, as well as the possibility of other ISO 639 parts becoming useful for the formation of language tags in the future.

The use and definition of private use tags has also been modified, to allow people to use private use subtags to extend or modify defined tags and to move as much information as possible out of private use and into the regular structure.

The goal for each of these modifications is to reduce or eliminate the need for future revisions of this document.

The specific changes in this document to meet these goals are:

Defines the ABNF and rules for subtags so that the category of all subtags can be determined without reference to the registry.

Adds the concept of well-formed vs. validating processors, defining the rules by which an implementation can claim to be one or the other.

Replaces the IANA language tag registry with a language subtag registry that provides a complete list of valid subtags in the IANA registry. This allows for robust implementation and ease of maintenance. The language subtag registry becomes the
canonical source for forming language tags.

Provides a process that guarantees stability of language tags, by handling reuse of values by ISO 639, ISO 15924, and ISO 3166 in the event that they register a previously used value for a new purpose.

Allows ISO 15924 script code subtags and allows them to be used generatively.
Defines a method for indicating in the registry when script subtags are necessary for a given language tag.

Adds the concept of a variant subtag and allows variants to be used generatively.

Adds the ability to use a class of UN M.49 tags for supra-national regions and to resolve conflicts in the assignment of ISO 3166 codes.

Defines the private use tags in ISO 639, ISO 15924, and ISO 3166 as the mechanism for creating private use language, script, and region subtags respectively.

Adds a well-defined extension mechanism.

Defines an extended language subtag, possibly for use with certain anticipated features of ISO 639-3.

Ed Note: The following items are provided for the convenience of reviewers
and will be removed from the final document.

9. References

9.1 Normative References

International Organization for Standardization, “ISO/IEC 10646:2003. Information technology -- Universal Multiple-Octet Coded Character Set (UCS), as, from time to time, amended, replaced by a new edition or expanded by the addition of new parts,” 2003.

Statistical Division, United Nations, “Standard Country or Area Codes for Statistical Use,” UN Standard Country or Area Codes for Statistical Use, Revision 4 (United Nations publication, Sales No. 98.XVII.9, June 1999.

Authors' Addresses

Appendix A. Acknowledgements

Any list of contributors is bound to be incomplete; please regard the
following as only a selection from the group of people who have contributed
to make this document what it is today.

The contributors to RFC 3066 and RFC 1766, the precursors of this document,
made enormous contributions directly or indirectly to this document and are
generally responsible for the success of language tags.

The following people (in alphabetical order) contributed to this document or
to RFCs 1766 and 3066:

Very special thanks must go to Harald Tveit Alvestrand, who originated
RFCs 1766 and 3066, and without whom this document would not have been possible.
Special thanks must go to Michael Everson, who has served as language tag
reviewer for almost the complete period since the publication of RFC 1766. Special thanks to Doug Ewell, for his production of the first complete
subtag registry, and his work in producing a test parser for verifying
language tags.

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